Biomechanical Validation and Torque Enhancement of a BLDC-Driven Active Ankle Prosthesis for Walking and Stair Locomotion

Background/Objectives: Lower-limb impairment can alter ankle-joint motion and reduce functional mobility during demanding locomotor tasks such as stair ascent and descent. This study presents the development and preliminary proof-of-concept evaluation of a BLDC-driven active ankle prosthesis incorporating a spherical gear coupling. Methods: Ankle kinematics were recorded using the CONTEMPLAS motion analysis system for one healthy participant and one participant with impaired lower-limb function during level walking, stair ascent, and stair descent. The resulting ankle-angle trajectories were used as reference profiles for unloaded bench testing of the prototype. Results: Static torque was measured using a handheld dynamometer and a fixed lever arm of 0.10 m. Adjustment of the BLDC controller parameters increased the measured static prototype torque from 0.44 N·m to 6.38 N·m, corresponding to a 14.5-fold increase relative to the initial controller configuration, without modification of the mechanical architecture. Under unloaded fixed-rig conditions, the prototype executed a prescribed cyclic ankle-angle trajectory within a range of 112–132°, corresponding to an amplitude of approximately 20°. The optimized prototype torque remained below the 17.17 N·m obtained from a preliminary static measurement of the healthy participant; however, this value should not be interpreted as a physiological torque requirement for dynamic locomotion. Conclusions: The results demonstrate a substantial control-based increase in the available static actuator output and the ability of the prototype to execute prescribed ankle-angle trajectories under controlled bench conditions. Further validation will require dynamic loading, kinetic measurements, foot–ground interaction, improved control strategies, and human-in-the-loop testing with individuals with lower-limb amputation.